The present invention relates generally to shift systems for power transfer devices and, more particularly, to a hydraulic shift system for a four-wheel drive transfer case.
With the advent of increased consumer popularity in four-wheel drive passenger cars and sport/utility vehicles, power transfer devices are more frequently being incorporated in vehicular driveline applications. As is known, part-time transfer cases are used in four-wheel drive vehicles for selectively directing power to the non-driven wheels upon shifting from the two-wheel drive mode to the four-wheel drive mode. More particularly, most part-time transfer cases use a transmission-type synchronized dog clutch for shifting "on-the-fly" from the two-wheel drive to four-wheel drive mode. The dog clutch can be actuated manually via a shift lever or it may be remotely actuated, as in an electrically-controlled shift system equipped with a power-operated actuator.
In addition, part-time transfer cases are also equipped with a two-speed planetary gearset for establishing high-range and low-range drive modes. Typically, a non-synchronized dog clutch is used for shifting between the high-range and low-range drive modes. An example of a part-time two-speed transfer case is disclosed in commonly-owned U.S. Pat. No. 4,770,280. Moreover, recent efforts have been made to provide "on-the-fly" shifting between the high and low-range drive modes. An example of a synchronized range shift system is shown in commonly-owned U.S. Pat. No. 5,346,442.
One drawback associated with conventional part-time transfer cases is that it is sometimes difficult, particularly during cold weather start-ups, to shift into the four-wheel drive mode. In manually-actuated part-time transfer cases, the vehicle operator may be required to exert a large input force on the shift lever to overcome the shift resistance associated with cold shifting into the four-wheel drive mode. Similarly, in electrically-actuated shift systems the electric motor must continue to provide a large input force during an extended shift period which, in some instances, may overload the electric motor. In other instances, the electric motor may generate an excessive amount of shifting force, causing the shifting process to be noticeably harsh and abrupt. As such, efforts have been made to reduce the shift effort and provide the vehicle operator with a means for smoothly shifting between the available modes. For example, commonly-owned U.S. Pat. No. 5,076,112 discloses an arrangement for reducing shift efforts in mechanically-operated shift systems. As a further example, commonly-owned U.S. Pat. No. 5,377,800 discloses a hydraulically-actuated mode shift system for a transfer case. While such systems are satisfactory for their intended purpose, a need still exists to continue development of alternative shift systems.